The present invention relates to methods for forming heat-resistant, semicrystalline articles from polylactic acid.
Polylactic acid (PLA) is useful for making films, fibers, and various types of formed articles. One limitation on its use in some food packaging and other applications is its tendency to deform when heated. For example, many food packaging applications require the resin to be subjected to the temperature of boiling water without significant deformation. PLA articles often cannot withstand such temperatures.
PLA tends to exist in an amorphous state when formed into these kinds of articles. Experience with a more conventional resin, poly(ethylene terephthalate) (PET) has shown that better resistance to heat can be obtained if the polymer has a greater amount of crystallinity. For PET, this has resulted in a specialized thermoforming process for inducing crystallinity into PET articles. In cPET thermoforming, a PET sheet is heated until soft enough to be formed, then transferred to a hot mold and formed under vacuum and pressure. The temperature and residence time in the mold are such that crystallites form in the resin. Once the needed crystallinity is obtained, the article is transferred into another mold of identical dimensions. This second mold is held below the glass transition temperature (Tg) of the PET resin, usually near room temperature. The colder temperatures “quench” the resin, “locking in” the as-formed dimensions. If the resulting PET resin is sufficiently crystalline, it may withstand use temperatures some 20-150 C higher or more than amorphous PET.
The PET thermoforming process has the drawbacks of requiring two molds, which increases capital investment and operating costs, and long forming times in the mold to allow crystallization to be completed, which reduces output per unit time and thus increases costs.
As mentioned, the cPET thermoforming process is a material-specific one that is designed around the particular characteristics of PET resin.
It has been recognized that PLA, like PET, can be formed into a more crystalline state by subjecting it to certain temperatures. See Kolstad, “Crystallization Kinetics of Poly(L-lactide-co-meso-lactide)”, J. Applied Polymer Science 62, 1079-1091 (1996). As described by Kolstad, the rate of crystallization is affected by various factors, including the lactic acid enantiomer ratio, the use of nucleating agents, and the thermal history of the polymer (i.e. time at crystallization temperature and/or cooling rates).
Nonetheless, no cost-effective commercial process has been developed for making formed crystalline PLA articles. It would be desirable to provide such a process, as it would permit PLA to be used in end-use applications that require improved heat resistance.